Field of the Invention
The present invention relates generally to recovery of carbon from alkaline water sources and, more specifically, to recovering carbon while simultaneously producing hydrogen gas.
Description of the Prior Art
The total carbon content of the world's oceans is roughly 38,000 GtC. Over 95% of this carbon is in the form of dissolved bicarbonate ion (HCO3−). Cline The Economics of Global Warming; Institute for International Economics: Washington D.C. (1992). This ion along with carbonate is responsible for buffering and maintaining the ocean's pH which is relatively constant below the first 100 meters. This dissolved bicarbonate and carbonate of the ocean is essentially bound CO2, and the sum of these species along with gaseous CO2, shown in equation 1, represents the total carbon dioxide concentration [CO2]T, of seawater.Σ[CO2]T=[CO2(g)]l+[HCO3−]+[CO32−]  (1)
At a typical ocean pH of 7.8, [CO2]T is about 2000 μmoles/kg near the surface, and 2400 μmoles/kg at all depths below 300 meters. Takahashi et al., “The Alkalinity and Total Carbon Dioxide Concentration in the World Oceans,” Carbon Cycle Modell., Vol. 16; SCOPE: NY, USA, pp 271-286 (1981); Takahashi et al., “Carbonate Chemistry of the Surface of the Waters of the World Oceans,” Isotope Marine Chemistry, Goldberg et al. eds., Uchida Rokakuho: Tokyo, Japan, pp 291-326 (1980). This equates to approximately 100 mg/L of [CO2]T of which 2 to 3% is [CO2 (g)] (equation 1), 1% is carbonate, and the remainder is dissolved bicarbonate. Johnson et al. showed that when the pH of seawater is decreased to 6 or less, the total CO2 exists only in the dissolved gas form. Johnson et al., “Coulometric TCO2 Analyses for Marine Studies: An Introduction,” Marine Chem., 16, 61 (1985).
There are methods for carbon capture in a sea-based application. Hardy et al., “Extraction of Carbon Dioxide From Seawater by Ion Exchange Resin Part I: Using a Strong Acid Cation Exchange Resin” NRL Memorandum Report, 6180-07-9044 (20 Apr. 2007); Willauer et al., “Recovery of [CO2]T from Aqueous Bicarbonate Using a Gas Permeable Membrane,” NRL Memorandum Report, 6180-08-9129 (25 Jun. 2008); Willauer et al., “Recovery of CO2 by Phase Transition from an Aqueous Bicarbonate System by Means of Multi-layer Gas Permeable Membranes,” Energy & Fuels 23, 1770-1774 (2009); Willauer et al., “The Effects of Pressure on the Recovery of CO2 by Phase Transition from a Seawater System by Means of Multi-layer Gas Permeable Membranes,” intended for Energy & Fuels (2009); Willauer et al., “Extraction of CO2 From Seawater By Ion Exchange Resin Part II: Using a Strong Base Anion Exchange Resins,” NRL Memorandum Report (2013); Willauer et al., U.S. Provisional Application No. 61/084,700 “Recovery of [CO2]T From Seawater/Aqueous Bicarbonate Using a Gas Permeable Membrane.” Efficiency can be an issue with these methods.
An electrochemical method to acidify seawater and recover [CO2]T simultaneously with hydrogen gas production from alkaline water sources including seawater has been developed at NRL. DiMascio et al., “An Electrochemical Method To Acidify Seawater and Recover CO2 Simultaneously with Hydrogen Gas from Alkaline Water Sources Such as Seawater,” U.S. Provisional Patent Application No. 61/333,553 filed May 11, 2010; DiMascio et al., “Extraction of Carbon Dioxide from Seawater by an Electrochemical Acidification Cell Part I: Initial Feasibility Studies” NRL Memorandum Report, 6180-10-9274 (23 Jul. 2010); Willauer et al., Willauer et al., “Extraction of Carbon Dioxide from Seawater by an Electrochemical Acidification Cell Part II: Laboratory Scaling Studies” NRL Memorandum Report, 6180-11-9329 (11 Apr. 2011); Willauer et al., “Development of an Electrochemical Acidification Cell for the Recovery of CO2 and H2 from Seawater,” Ind. Eng. Chem. Res., 9876-9822 (2011); Willauer et al., “Extraction of Carbon Dioxide from Seawater by an Electrochemical Acidification Cell Part III: Scaled-Up Mobile Unit Studies (Calendar Year-2011)” NRL Memorandum Report, 6300-12-9414 (30 May 2012).